Lignite, often referred to as brown coal, is the lowest rank of coal and is used almost exclusively as fuel for steam-electric power generation. It is brownish-black and has a high inherent moisture content, sometimes as high as 66 percent, and very high ash content compared with bituminous coal. It is also a heterogeneous mixture of compounds for which no single structural formula will suffice. Lignite has characteristics that put it somewhere between black coal and peat. When dried, it crumbles easily. It is found in abundance in many areas around the world including the United States, Australia, Canada, Greece and Germany where it is generally used as a fuel for generating electricity. A significant amount of the electricity generated in those countries comes from the combustion of lignite.
The potential heat content of lignite varies widely depending on the moisture content, the source and the combustion technology. Because of its low energy density, brown coal is inefficient to transport and is not traded extensively on the world market compared with higher coal grades, such as black coal. Brown coal is often burned in power stations constructed very close to any mines, such as in Australia's Latrobe Valley. These mines are rarely underground due to the location of the coal close to the surface, and strip mining is usually practiced. Carbon dioxide emissions per unit power generated are generally much higher from brown coal fired plants than comparable black coal plants. The moisture content of lignite normally necessitates drying of the feed prior to combustion in a power station, which adds to the cost and complexity of the process.
The continued operation of brown coal plants, particularly in combination with strip mining, is widely criticised on environmental grounds. As a result there is a need to find ways of generating high value product fuel stream(s) from a lignite deposit.
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